As historians of law have long recognized, Gratian's Harmony of Discordant Canons, commonly known as the Decretum, is a foundational text for modern Western jurisprudence. Even though this text presents many interpretative difficulties, in part because it has almost certainly been altered and expanded in a variety of ways, its main purpose is clear enough. That is, as the title indicates, it represents an attempt to show how the diverse and seemingly inconsistent canons of the church can be interpreted and applied in a consistent way – in a word, harmonized. In the first twenty distinctions of the Decretum Gratian sets forth the general principles governing the interpretation of laws. (Thus, modern commentators often refer to these distinctions as the “treatise on laws,” a usage I follow.) While these distinctions do not comprise a fully worked out theory of law, they do set forth a coherent conception of law developed in what was to become the preferred scholastic way – that is to say, through the selection and interpretation of authoritative texts, here including copious citations from patristic sources and Isidore's Etymologies as well as legal texts properly so called.

The Decretum begins as follows: